Certain ceramic materials and inorganic crystals, such as quartz and barium titanate, have been known to exhibit piezoelectric characteristics. Piezoelectric materials transform a mechanical force to an electrical response and, conversely, transform an electrical signal to a mechanical motion. If alternating compressive and tensile stress is applied, opposite electrical charges are produced on opposing faces. When an electrical charge or potential difference is applied to piezoelectric material, it undergoes changes in thickness and thus produces mechanical forces. If alternating voltage is applied, periodic variations in thickness are produced. Conventional piezoelectric materials are hard, stiff, brittle, dense, and difficult to process. They are used in devices such as loudspeakers and clocks.
Recently, certain organic polymers, such as polyvinylidene fluoride (PVF.sub.2), polyvinyl fluoride and polyvinyl chloride have been shown to exhibit piezoelectric properties. Such polymers can be processed into films which are flexible and lightweight. PVF.sub.2 is a semi-crystalline high molecular weight polymer whose basic building block is (CH.sub.2 --CF.sub.2). When properly treated by orientation and polarization, PVF.sub.2 films have high piezo- and pyroelectric activities.
After polarization, a piezoelectric film has a positive side and a negative side. When positive voltage is applied to the positive side of the film, it causes the film to elongate. Conversely, negative voltage applied to the positive side causes the film to shrink. Alternating voltage causes oscillations of the film which are too small to be visually detected. Larger strains can be produced through the use of a bimorph, which consists of two piezoelectric films glued together along their positively charged sides. Voltage applied to the bimorph will cause the top film to elongate and the bottom film to shrink. Such movement can be seen by the naked eye if an alternating voltage is applied at appropriate frequencies.
Very thin films (down to a few microns) can be made from the PVF.sub.2. To date, PVF.sub.2 films have found numerous commercial uses. These include use in sensing devices, microphones, audio component membranes, underwater sounding detectors, medical diagnostic equipment, pressure sensitive elements in contactless switches and manual keyboards and nondestructive materials testing methods.
A necessary consideration in designing a mechanical system is the control of vibrations, which occur in all moving mechanical systems. If left undamped, vibrations can cause large dynamic stresses which in turn lead to fatigue failure. Fatigue failure not only reduces the useful life of the mechanical system, but also may cause breakdown or failure to occur at a critical point in its operation. Additionally, in some robotic applications, the response time can be considerably improved if vibrations can be damped more quickly.
One technique which has been used in efforts to damp or check vibrations is the application of a viscoelastic material applied to a vibrating surface. A viscoelastic material is one which is viscous but which also exhibits certain elastic properties such as the ability to store energy of deformation. In such a material, the application of a stress gives rise to a strain that approaches its equilibrium value slowly. In a modification of this technique, a stiff foil is placed on top of the viscoelastic material, thus creating a constrained viscoelastic damper. This stiff constraining layer causes a large shear to occur in the viscoelastic material. As a result, more energy is dissipated per cycle of vibration and damping of the vibration occurs more rapidly than would be the case without the constraining layer. Such a viscoelastic layer with a constraining foil is manufactured in tape form. Presently, this configuration is used as a noise-control measure for aircraft fuselages and in damping the sway of skyscrapers.
The design of the constrained layer damper is determined by the desired application, the frequency of resonance of vibration, temperature requirements and weight and size considerations. If materials are carefully selected, they will provide a good damping effect for a wide range of applications. Weight considerations are often important in material selection and ideally, a damper will provide the required damping with a minimum of added weight.
Vibration control is often critical in aircraft, robotics, and satellites. In space, the need for vibration control is especially problematic because there is no natural damping effect other than the inernal damping present in the structure itself. This problem may be compounded by the fact that some satellites have flexible long arms with a low natural frequency and thrusters at points. Control of such a flexible beam is difficult. A damping device able to act at many points along a flexible beam is desirable in this context and for many other control applications.
The applicability of a constrained viscoelastic damper in such a context has been investigated. For example, a constrained layer damper has been built into beams of various designs. Inserts of the viscoelastic damping material are placed in the beam. A large amount of shear is created in the inserts as the beam vibrates. Localized application of damping tapes to the surface is another approach which has been tested as a means of providing damping along a flexible beam. In addition to allowing damping along such a beam, viscoelastic inserts and localized application of damping tapes reduce vibrations with a smaller weight increase than would occur if the entire surface of a vibrating part were covered.
Application to less than the entire surface requires a full understanding of the vibration modes if localized damping is to be effective in relation to all frequencies of vibration and thus in providing adequate vibration reduction. In addition, the damping capabilities of a constrained layer have been shown to be adversely affected by dissipative heating.